Precious Stones: Physical Properties of Gemstones Page 46



	PRECIOUS STONES. 41 be seen in most rough precious stones ; it must be distinguished from cleavage (q.r.) ; fractured surfaces, though they approach geometrical planes in some cases, are never true planes. "When the fracture is flat or nearly flat it is said to be " even " ; this is seen in some Jaspers. Should it be rougher and covered with minute points it is " uneven," as in Lapis Lazuli; when still more rough it is "hackly." When the broken surface shows the smooth curves so well seen in a broken piece of thick glass, the fracture is " conchoidal" (i.e. shell-like), this is well seen in Quartz and many of the gem stones. Where a gem stone has been damaged, as by a blow, but without separation of the fragments, a crack may often be seen reflecting beautiful prismatic colours (c.f. refraction). This is occasionally made use of in stones that have no intrinsic beauty of colour as in Quartz; but in a highly refracting gem any flaw of the sort, especially if situated at the back part of the cut stone, greatly detracts from the brilliance, so that when a flaw develops before the stone is cut, the size of the finished stone and the direction of the cuts are arranged- to eradicate such blemishes. This was exemplified in the case of the Koh-i-noor, which as exhibited at the Crystal Palace weighed 186-1/16 carats, but which was afterwards re-cut by Messrs. Garrard, when it weighed 102-1/4 carats. Brittleness depends very largely on the grosser structure of the stone ; where this is minutely fibrous, as in Crocidolite or Malachite, there may be considerable tenacity, though the hardness of these two minerals is very different; at the same time the brittleness depends partly on the hardness, and also on the presence or absence of a cleavage.